can't imessage to other iphone users - going to SMS

Have been having odd problems with iPhone4 through Verizon. Using iOs5.1.1.

Until mid February, I had been able to send free text messages between myself and my friends who had been using iPhones. You could tell they were free because they were blue text bubbles.

Now some of the same friends and I can still extend free iMessages between our iPhones - blue text bubbles. But with other friends who have iPhones the text bubbles are green. Sometimes with the same friend on the same day some text bubbles are green and some are blue. It's not making any sense.

Are there problems with Apple iMessage causing these problems? I don't think it's my older iOs because I clearly am able to send free blue bubble text messages between some friends - just not others.